Fix ubsan failures (pass NULL to memcpy)

lib/decompress/zstd_decompress.c

@@ -1402,7 +1402,9 @@ ZSTD_DDict* ZSTD_createDDict_advanced(const void* dict, size_t dictSize, ZSTD_cu
             return NULL;
         }
 
-        memcpy(dictContent, dict, dictSize);
+        if (dictSize) {
+            memcpy(dictContent, dict, dictSize);
+        }
         {   size_t const errorCode = ZSTD_decompressBegin_usingDict(dctx, dictContent, dictSize);
             if (ZSTD_isError(errorCode)) {
                 ZSTD_free(dictContent, customMem);